Using Waste Heat in Controlled Environment Agriculture

Rooftop solar thermal is a suitable sustainable heat source, in addition to waste heat

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Controlled Environment Agriculture (CEA), such as vertical farming and greenhouse production, enables highly efficient food production using less land and water than field farming, while delivering reliable year-round yields. In addition to the many other benefits, the ability to build and operate CEA facilities nearly anywhere in the world enhances food security and reduces food miles by enabling food production closer to population centers. However, to maintain optimal growing conditions, CEA requires significant energy, particularly due to high HVAC loads and artificial lighting. Thankfully, this agricultural method has many opportunities and technologies that allow for significant reductions in energy generation by reducing energy inputs and using alternative energy sources (Engler & Krarti, 2021).

Many strategies exist to reduce the energy footprint of CEA, such as increasing the facility/greenhouse envelope’s insulation, or implementing high efficiency HVAC systems. Another approach is to reduce reliance on purchased energy by integrating alternative energy sources such as renewable energy or recovered waste heat. AgriTech North’s Hydronic Microclimate Control System is an example of this approach, using alternative energy, such as waste and renewable thermal energy to deliver heating, cooling, and dehumidification with significantly lower energy consumption than conventional systems. This paper introduces the concept of waste heat, methods of waste heat recovery and how CEA can benefit from using previously discarded thermal energy.

Waste heat

Waste heat is thermal energy produced as an unintended byproduct of energy conversion processes. Because no energy conversion system is perfectly efficient, a portion of the input energy is inevitably transformed into less useful forms. This loss most commonly appears as heat generated through mechanical friction, electrical resistance, or nuclear and chemical reactions. When this thermal energy is not needed for the intended purpose of the system and is instead rejected to the environment, it is referred to as waste heat.

To understand waste heat, it is helpful to consider how frequently energy is converted from one form to another (Fig 1). In modern society, energy conversion enables nearly all aspects of daily life, from powering homes and transportation to enabling food production and manufacturing. While all energy conversions produce some degree of waste heat, its significance depends on the scale and purpose of the system, with the relevance and impact of waste heat increasing alongside the size and intensity of the energy conversion process.

*Figure 1 - Examples of daily energy conversions*

The systems responsible for converting energy from one form to another to perform useful work are known as energy conversion devices (Radovic, n.d.). Many of the largest of these systems, such as power plants and combustion engines, convert chemical or nuclear energy into useful motion or electricity. However, these systems are also among the least efficient forms of energy conversion. Their efficiency is defined as the fraction of the input energy that is converted to a useful output, with the remaining energy classified as waste energy.

The definition of “useful” energy depends on the intended purpose of the system. For this reason, a system which makes use of the thermal energy that it produces has an increased efficiency compared to an equivalent system which rejects its waste heat. The technology employed to make use of the thermal energy produced in large energy conversion systems, such as power plants and combustion engines, is Combined Heat and Power (CHP) generation. CHP involves simultaneously producing electricity and thermal energy from a single fuel source, greatly increasing the system’s energy efficiency and reducing heating costs for its thermal energy recipients. For example, in a case study on a gas turbine power plant, it is demonstrated that incorporating waste heat recovery, by operating as a CHP plant, increases its’s efficiency from approximately 33.66% to 50.20%, while also raising power output from about 57.5 MW to 85.7 MW using the same fuel input (Saturday, & Yahaya, 2025).

*Figure 2 – Combined Heat and Power diagram*

Cars are another example of inefficient energy conversion systems. Only a small portion of a car’s input energy is converted to useful output with a typical internal combustion engine operating at about 20-25% efficiency, with roughly 60-70% of the fuel’s chemical energy being released as waste heat (Vasta, 2023). As shown in the energy diagram in Figure 3, only a fraction of the energy consumed by the engine is converted to mechanical work, while the majority is lost through exhaust gases and engine cooling systems.

*Figure 3 - Energy flow Sankey Diagram for an average car engine*

Because waste heat is an unavoidable byproduct of energy conversion, removing this heat becomes a critical requirement for nearly all machinery and electrical systems. This often requires additional energy input, further contributing to the total energy consumption. In many cases, the energy used for cooling and heat removal represents a significant portion of the system’s overall energy use, as shown in the coolant energy consumption in Figure 3. The magnitude of energy required for waste heat removal is highlighted by high heat producing systems like data centers, where cooling systems can account for up to 40% of total energy consumption. By implementing waste heat recovery, energy conversion systems not only improve efficiency but can also greatly reduce cooling loads associated with expelling waste heat.

In most applications, waste heat is simply expelled into the environment despite containing significant amounts of usable energy. A common example is fossil-fueled plants, which reject excess heat into bodies of water or into the atmosphere through cooling towers, steam plumes and exhaust gases. In the United States, the average energy efficiency of fossil-fuelled power plants is 36%, meaning most of its input energy is lost to the environment as heat. Not only is this wasteful, but it is thermal pollution, which leads to the destruction of natural ecosystems inhabiting these bodies of water or the plant’s surrounding area.

In many cases, this thermal energy can be recovered and reused for heating or further energy conversion processes. By redirecting waste heat, overall energy consumption can be reduced, system efficiency can be improved, and a low-cost, sustainable energy source can be created for secondary applications.

Waste heat recovery

This leads to the concept of waste heat recovery, which refers to processes used to capture and reuse thermal energy that would otherwise be rejected to the environment. Waste heat recovery systems either redirect this thermal energy to an application where it can be used directly or convert it into another useful form. Direct heat reuse is the most common approach, as it is the simplest and most cost-effective method, avoiding additional energy losses from further conversion processes.

There are different waste heat recovery systems depending on the temperature of the available waste heat. At temperatures over 400°C, high-grade waste heat is typically produced by direct combustion processes present in power generation and industrial manufacturing. At these higher temperatures, recovery is more effective due to the greater energy content of the heat. An example of this is regenerative and recuperative burners, used in metal and glass furnaces, where hot exhaust gas transfers heat to a heat-resistant medium that preheats incoming combustion air, reducing energy demand for the process. Medium-grade waste heat (100-400°C) is commonly found in combustion exhaust streams and is often used to boil water to be used for heating as well as producing electricity through further energy conversions. Low-grade waste heat (below 100°C), the most abundant form, originates from sources such as data centres or machinery cooling (Jouhara et al., 2018). Although low-grade waste heat is more difficult to utilize due to its lower energy content, its continuous availability and low temperature are well suited for applications such as district or CEA facility heating. It can also be converted into other useful outputs such as cooling and electricity (Ling-Chin et al., 2018). The recovery of low-grade waste heat is common practice in rural and remote areas in the form of district heating. An example of this is the recovery and use of thermal energy produced by electricity generators in communities in Nunavut, which has resulted in 10% reduction in heating costs for their customers as well as a reduction equipment maintenance and carbon emissions.

When direct use of thermal energy is not feasible due to factors such as temperature, location, or timing, waste heat can instead be converted into other forms of energy. This is often achieved using thermodynamic cycles, such as the Rankine cycle, in which waste heat is used to vaporize a working fluid, expanding it through a turbine producing mechanical power and electricity (Oluleye et al., 2016). Another important example is the use of adsorption chillers, which convert low grade waste heat to produce cooling. This technology allows systems that already produce waste heat to meet part of their cooling demand internally, reducing dependence on conventional, energyintensive cooling technologies and improving overall energy efficiency of the system. This technology is ideal for warmer climates/seasons where waste heat is not desired and can instead be converted into much needed cooling.

Waste Heat Recovery in CEA

While waste heat recovery has traditionally been applied to buildings and industrial systems, its characteristics align particularly well with the needs of Controlled Environment Agriculture. CEA facilities are inherently energy-intensive, requiring continuous and precise control of temperature, humidity, lighting, and airflow to optimize plant growth. This makes them ideal recipients for waste heat and well positioned to recover and reuse their own waste heat. By making use of water-based heating and cooling, CEA facilities can allow for thermal energy produced by energy conversion devices to be captured and used efficiently elsewhere in the facility. AgriTech North’s Hydronic Microclimate Control System allows for this by recovering thermal energy and using it to supply heating, cooling and dehumidification enabling a great reduction in energy costs.

The energy consumption of CEA results in substantial amounts of waste heat production from equipment such as LED lighting, chillers, and dehumidifiers. Typically, this waste heat is vented out or cooled, requiring additional energy to do so. By implementing waste heat recovery, enhanced by thermal storage, hydronic heating and adsorption cooling, CEA facilities can greatly improve their efficiency by reusing energy, and cut down on energy consumption. A case study of a 25-acre tomato greenhouse demonstrates the benefits of integrating waste heat recovery with thermal storage, which resulted in a 18.94% decrease in energy procurement cost and a 24.34% reduction in operational emissions (Seiler et al., 2025).

Because CEA facilities run continuously, demand large amounts of energy and can use both high and low-grade thermal energy, they are uniquely well-suited to utilize external waste heat streams that might otherwise be rejected into the environment. This creates opportunities for industrial symbiosis, where waste heat from one process is used as an energy input for another, leading to mutual economic benefits and reduced carbon emissions. Data centres are examples of energy conversion systems that benefit greatly from a symbiotic relationship with CEA facilities. Nearly all energy used by data centres is released as low-grade heat. By redirecting this heat to be used in CEA, data centres can greatly reduce cooling loads, reduce emissions, and provide inexpensive, constant thermal energy to CEA operations. An example of this type of symbiotic relationship is Truly Green Farms in Chatham-Kent, Ontario, which utilizes waste heat and CO2 from a nearby ethanol plant. By locating the greenhouse adjacent to the plant, a heat exchanger is used to capture waste heat at 65-70 °C and transfer it to the greenhouse in the form of hot water to provide heating to the facility. Spanning 90 acres, Truly Green Farms’ greenhouse has a minimal carbon footprint by virtually eliminating heating and CO2 input, while reducing the ethanol plant’s emissions.

Cooling and dehumidification represent the second-largest energy demand in CEA systems after artificial lighting. Heat from artificial lighting, solar radiation, and facility equipment requires continuous cooling to maintain optimal growing conditions. In parallel, humidity control is critical, as plant transpiration releases moisture into the air, increasing indoor humidity levels. Both processes require the generation of chilled water that is typically provided by vapor-compression based systems, such as AC units and compression chillers. Adsorption chillers are an alternative to vapour-compression based cooling, which is typically powered by electricity and uses refrigerants as its working fluid. Instead, adsorption chillers are powered by low-grade thermal energy and use water as their working fluid (Fig 4), allowing integration with various waste heat recovery methods due to the abundance of low-grade waste heat sources. In addition, the elimination of high global warming potential refrigerants reduces environmental impact as well as allowing for self maintenance. These characteristics make adsorption chillers well suited for CEA applications, particularly in remote settings, which is why AgriTech North’s Hydronic Microclimate Control System uses this technology to deliver cooling and dehumidification.

The benefits of waste heat for CEA are even more evident in cold climates/seasons. In these situations, space heating represents a major portion of CEA’s operating energy demand, especially in greenhouses, which have significantly lower insulation compared to conventional buildings. As a result, in colder climates, greenhouse production is often limited to the warmer months of the year due to the high energy costs of heating. Integrating CEA systems with external waste-heat sources such as data centres, generators and power plants can allow for these systems to provide low-cost thermal energy, enabling year-round production thanks to the greatly reduced energy costs. For this reason, Combined Heat and Power (CHP) plants are commonly used to heat CEA facilities in cold climates, particularly in European countries where CHP is becoming a common energy solution for commercial greenhouse operations. An example of this is the Boden greenhouse project in Sweden, which uses waste heat from a nearby data centre to power year-round food production in a subarctic climate. Similarly, RegEnergy Frövi operates a 10-hectare greenhouse heated entirely by waste heat from a paper mill, supplemented by hydropower, resulting in an 83% reduction in global warming impact for its production of tomatoes.

The recovery and use of waste heat has shown to be an ideal energy source for CEA, both on a large scale such as industrial symbiosis between greenhouses and power plants, but also on a smaller scale, such as a CEA operation capturing and reusing waste heat generated on site. Regardless of if operations are in cold or warm climates, substantial amounts of waste heat can be recovered and used for heating through direct heat reuse or cooling, dehumidification and even electricity powered devices through further energy conversions. As energy costs continue to rise and CEA facilities look to reduce emissions, CEA operators are looking for energy efficient and sustainable solutions to maintain ideal growing conditions. Waste heat continues to demonstrate that it can be a reliable energy source for these facilities going forward.